1) Field of the Disclosure
The disclosure relates generally to glass preforms used in fiber drawing, and more particularly to glass preforms with nanofiber reinforcement.
2) Description of Related Art
Composites consisting of glass or carbon fibers in a plastic matrix are used in a wide variety of mechanical structures. It is desirable to use fibers with high tensile strength and modulus in these composites as this enables the structure to be made lighter and often less expensively. It is known in the art that glass fiber with carbon nanofiber (CNF) or carbon nanotube (CNT) reinforcement provides a higher strength fiber than glass fiber by itself. Typically, the glass fiber has a diameter of 10 microns to 20 microns and lengths of many meters. The nanofiber reinforcement might have a diameter of 100 nm (nanometer), and a length of 20 micron. The volume fraction of nanofiber in the glass fiber might be approximately in the range of 0.01% to 50% percent. Carbon nanotubes might have a diameter ranging from 2 nm to 50 nm and a length of 1 micron to 20 microns. For the reinforcement to be effective over the entire length of the glass fiber, it is important that the reinforcing nanofibers be oriented approximately along the length of the fiber and that their distribution be substantially uniform. Carbon nanotubes have electrical polarizabilities in the transverse direction to the fiber length, but the values in this direction are typically an order of magnitude or more smaller than the values along the fiber length.
Methods of drawing glass fiber from a preform are known. A known method of drawing fibers with nanofiber reinforcement is to disperse the nanofibers in a glass preform. The glass preform is then heated such that the glass viscosity becomes suitable for drawing into a fiber only at the very bottom of the preform. In drawing glass fiber, it is important that any trapped gas be removed from the preform during preform fabrication. This can be handled conveniently by degassing the melted glass in a vacuum chamber. However, when nanofibers are dispersed in the preform, the density of the nanofibers is generally different from that of the glass. In the time that it takes for the trapped air to degas, the nanofibers can either float to the top of the molten glass by buoyancy force or can sink to the bottom by the force of gravity.
Known methods exist for using electric fields to orient carbon nanotubes during their growing process. Carbon nanotubes are electrically polarizable, and polarized carbon nanotubes tend to line up in the direction of the electric field. Because of the small size of the carbon nanotubes, they can undergo substantial oscillation at temperature in gas, and electric fields of the order of one (1) Volt per micron are required for orientation. In known processes, nanotubes can exist in a molten glass with a substantial viscosity, compared to gas, that will tend to damp thermal vibrations. Thus, considerably less electric field strengths are expected to be useful for orienting the nanotubes in molten glass than are used in their growth process. In addition to using an electric field to orient the nanofibers, an electric field gradient can be used to apply a body force to the nanofibers to control the positioning of the nanofibers in a glass melt. There is a tendency of the nanofibers to form a network in the presence of the electric field, i.e., the positive charge of the end of one fiber will attach to the negative charged end of another fiber. Through this process, the homogeneous distribution of nanofibers throughout a glass preform will be assisted.
If an object is electrically polarized, a body force can be exerted on it by placing it in an electric field gradient. As an example, for a 2-nm diameter, 20 micron-long nanotube, the electrical polarization constant along the length of the fiber is A=2e-23 Nm^3N/V^2. Assuming that the difference in density between the molten glass and the nanotube is 1000 kg/m3, there will be a buoyancy force on the fiber of 6e-19 N. The force from the electric field gradient is F=2 A E dE/dz, where E is the electric field and dE/dz is the field gradient. With an electric field of 1e4 V/m and a field gradient of 1 V/m^2, the electric force on the fiber is 4e-19 N. Thus, modest values of electric field and electric field gradient are sufficient to control the buoyancy force. The polarization varies according to the square of the fiber length and is proportional to the fiber radius. In this case, even fiber diameters of 100 nm can easily be accommodated by modest electric field and field gradient values. Further, this will have the effect that only fibers of a particular length will establish a force equilibrium. This tendency can be partially mitigated by allowing the field gradient to vary over the length of the melted glass. This can distribute fibers of different length along the length of the molten glass. In addition, fibers outside of a design range and extraneous debris can either float up or sink down in the glass melt. This contamination can be removed after the glass has cooled by removing a fraction of material from the preform at the top and bottom.
Accordingly, there is a need in the art for a device and method for glass preforms with nanofiber reinforcement that provide advantages over known devices and methods.